The phrase "greed versus grievance" or "greed and grievance" refers to the two baseline arguments put forward by scholars of armed conflict on the causes of civil war, though the argument has been extended to other forms of war, such as violent conflict in general, rebellion and insurgency, for example.
"Greed" is shorthand for the argument that combatants in armed conflicts are motivated by a desire to better their situation, and perform an informal cost-benefit analysis in examining if the rewards of joining a rebellion are greater than not joining. "Grievance" stands for the argument that people rebel over issues of identity, e.g. ethnicity, religion, social class, etc., rather than over economics. In practice, even proponents of strong versions of these arguments admit that the opposing argument has some influence in the development of a conflict.
Whether the cause of war is attributed to 'greed' or 'grievance' the common factor is the perception of a certain deprivation. If it is an economic deprivation, the inequality will be a 'vertical inequality' and the cause of war will be 'greed'. If the deprivation is caused by ethnicity, age, religion or gender, it will be a 'horizontal inequality' and the cause of war will be due to the 'grievances'.